1. Field of the Invention
The present invention relates to an interference canceler, and, more particularly, to a method and apparatus for cancelling interference signals by using a power inversion adaptive array (hereinafter called the "PIAA").
2. Description of the Related Art
A diversity receiver is used typically in order to compensate for effects of multipath-fading phenomena in a long hop microwave communication. FIG. 7 shows a general block diagram of conventional microwave communication systems with dual type space diversity. In FIG. 7, the left side is defined to be a first radio station, and the right side is defined to be a second radio station.
In the first radio station, 201 is a transmitting baseband circuit (TX BB) for processing baseband signal TX DATA. 202 is a digital modulator (MOD) for modulating an output of the TX BB 201. 203 and 204 are two transmitters (TX) for transmitting the modulated signal. 205 and 206 are two antennae for transmitting outputs of the transmitters 203 and 204 to the second radio station. 207 and 208 are two receivers (RX) for receiving transmitted signals from the second radio station. 209 is a PIAA circuit for cancelling interferences. 210 is a digital demodulator (DEM) for demodulating the modulated signal. 211 is a decision unit (DEC) for recovering TX DATA. 212 is a receiving baseband circuit (RX BB) for processing the recovered data.
As the second radio station has the same configuration as the first radio station, explanation of the second radio station is omitted.
Transmitted data signal (TX DATA) of the first radio station is added with overhead signals such as a frame synchronization signal by the transmitting baseband circuit 201, and subjected to time division multiplex (TDM) and speed conversion. Output of the transmitting baseband 201 is modulated by the modulator 202, i.e., frequency modulated from an intermediate frequency (IF) band to a radio frequency (RF) band, and power amplified, branched into two signals, and input into the transmitters 203 and 204. Outputs of the transmitters 203 and 204, S, are supplied to the antennae 205 and 206. The first radio station is assumed to transmit the RF signal in vertical polarization (VP). The RF signal transmitted from the first radio station is spatially spread and propagated in a radiation pattern of the antenna.
It is assumed that (i) a propagation constant (a complex number): from the antenna 205 of the first radio station to the antenna 225 of the second radio station is h11, (ii) a propagation constant from the antenna 205 of the first radio station to the antenna 226 of the second radio station is h12, (iii) a propagation constant from the antenna 206 of the first radio station to the antenna 225 of the second radio station is h21, and (iv) a propagation constant (complex number) from a antenna 206 to the antenna 226 of the second radio station is h22.
In general, microwave communication has a problem that various interference signals degrade quality in a communication link, or makes it impossible to perform communication. Examples of such interference signals include interference from an adjacent channel, radar interference, or a disturbing signal.
In FIG. 7, 240 designates an interference signal source J, which is assumed to be received by the antennae 225 and 226 of the second radio station. It is assumed that a propagation constant (a complex number) from the interference signal source J to the antenna 225 is J1, and a propagation constant from the interference signal source J to antenna 226 is J2. Since the plane of transmission polarization at the first radio station is vertical polarization (VP), the plane of receiving polarization at the second radio station uses the same vertical polarization.
In the second radio station, received signals from the antennae 225 and 226 are input into the receivers 227 and 228, respectively, and subjected to processing for low noise amplification and frequency conversion from the RF band to the IF band. If received signals output from the receivers 227 and 228 are r1 and r2, they can be expressed as follows: EQU r1=h11.multidot.S+h21.multidot.S+J1.multidot.J (1) EQU r2=h12.multidot.S+h22.multidot.S+J2.multidot.J (2)
where S indicates a transmitted signal, and is a desired signal at the receiver side.
The two-branch received signal is linearly combined by the PIAA circuit 229. The PIAA circuit 229 uses a technique conventionally implemented to cancel interference signals.
Output y of the PIAA circuit 229 is expressed as follows: EQU y=W1.multidot.r1-W2.multidot.r2 (3)
When equations (1) and (2) are substituted for equation (3), the following is obtained: EQU y={W1(h11+h21)-W2(h12+h22)}.multidot.S+(W1.multidot.J1-W2.multidot.J2).mult idot.J (4)
Here, the requirement for the interference component J to be canceled is: EQU W1.multidot.J1-W2.multidot.J2=0 (5)
The PIAA circuit 229 finds the following as a solution of (5): EQU W1=1/J1 (6) EQU W2=1/J2 (7)
and performs linear combination to cancel unnecessary interference signals.
The PIAA circuit shows diversity combination of two branches. Another prior art system, a general system using an N-element antenna array is published by Compton in "The Power Inversion Adaptive Array: Concept and Performance", IEEE Transaction on Aerospace and Electronics Systems, Vol. AES 15, No. 6, November 1979.
In a PIAA, it is difficult to find weight coefficients W1 and W2. The Compton system finds the weight coefficient from correlation between an error signal, between an array combiners output and a reference signal, and each branch received signal. However, there is no detailed description on the reference signal. Thus, when it is intended to be implemented as a device, there arises a problem as to what is employed as the reference signal.
FIG. 8 shows an example of a conventional interference canceler in which the PIAA is applied to a diversity receiver. 301 and 302 are automatic gain control amplifiers (AGC) in each branch. 303 and 304 are complex multipliers for multiplying a weight coefficient by the outputs of the AGC amplifiers 301 and 302. 305 and 306 are correlaters (CORR) for obtaining the weight coefficients W1 and W2 of respective branches. 307 is a subtractor, 308 is an adder, 309 is an AGC amplifier (AGC), and 310 is a switch (SW).
In FIG. 8, diversity combination is performed by the adder 308, and its combiner system is a maximum ratio combiner. That is, first, level fluctuation of flat fading is removed by the AGC amplifiers 301 and 302 at each diversity input. Then, the multipliers 303 and 304 multiply the complex weight coefficients W1 and W2 so that the maximum ratio combination is performed by the adder 308. These weight coefficients are found by the correlaters 305 and 306 from correlation between the output of the AGC amplifier 309 after diversity combination and the outputs of AGC amplifiers 301 and 302.
If there exists no interference signal, the switch 310 selects and outputs the output of the AGC amplifier 309. If there is strong interference signals that makes the D/U ratio (ratio of desired signal to the undesired interference signal) negative, the switch 310 selects and outputs the output of the subtractor 307. The subtractor 307 subtracts the output of the complex multiplier 304 from the output of the complex multiplier 303, and cancels the interference signal through inverse-phase combiner on phase, while the adder 308 performs in-phase combiner on phase.
The operation of the conventional interference canceler is described with reference to FIG. 9A-9L. FIG. 9A and 9D show inputs (Input 1 and Input 2) of the diversity branches 1 and 2, respectively. Here, it is assumed that S1 and S2 are desired signals of each branch, and their interference signals are J1 and J2. If the interference signal is so strong as to makes D/U negative, the interference signal is normalized by the AGC amplifiers 301 and 302.
While, in the initial state, phases of interference signal vectors of the multipliers 303 and 304 are not in-phase, and most output components at the adder 308 are interference signals. When normalized by the AGC amplifier 309, and correlated with outputs of the AGC amplifiers 301 and 302, phase information of the interference signal vector of each branch can be obtained in a complex conjugate form as reference of an interference signal at the output of the AGC amplifier 309, as weight coefficients W1 and W2 of each branch. Multiplication of them in each branch can control the interference signal vectors in-phase to the phase of the normalized interference vector of the output of the AGC amplifier 309.
That is, the adder 308 can combine the interference signals in-phase to each other. Thereby, as shown in FIGS. 9B and 9E, amplitudes and phases of the interference signal outputs J1 and J2 at the multipliers 303 and 304 are controlled to be equal respectively. FIG. 9C shows in-phase combination of the interference signals J1 and J2 at the output of the adder 308. As shown in FIG. 9F, since the interference signals J1 and J2 are cancelled and eliminate each other at the output of the subtractor 307, only the desired signal is extracted.
The PIAA output, having cancelled the interference signals is input into the demodulator 230, and then demodulated. The demodulated signal is in a state that an eye pattern is bandwidth-limited. The demodulated signal is determined by the decision unit 231 and recovered as a digital signal. The recovered digital signal is input into the receiving baseband circuit (RX BB) 232, where it is subject to frame synchronization or the like, so that signals such as an order wire signal are separated and extracted.
The above description is for transmission of a signal from the first radio station to the second radio station. Alternatively, a signal may be transmitted in the same manner from the second radio station to the first radio station. However, in this case, horizontal polarization (HP) is employed for the plane of polarization for a radio signal transmitted from the second radio station to the first radio station.
When the interference signal is cancelled for the PIAA circuit in the prior art described above, the desired signal may be also lost depending on the phase relationship of the desired signal input into each array branch of the PIAA circuit.
That is, if the desired signal S and the interference signal J are in the same amplitude and phase relationship for the inputs 1 and 2 as in FIG. 9G and 9J, the outputs of the multipliers 303 and 304 agree with each other as in FIG. 9H and 9K. At that moment, the output of the adder 308 is in-phase combined for the desired signal S and the interference signal J as in FIG. 9I. On the other hand, the output of the subtractor 307 is inverse-phase combined for both the desired signal S and the interference signal J in FIG. 9L. Accordingly, the output of the subtractor 307 is cancelled for the interference signal, but the desired signal is also cancelled. This corresponds to a case where the coefficient for the desired signal S is zero in the first term in the right side of equation (4). That is, if EQU W1(h11+h21)=(h12+h22) (8)
the desired signal disappears.